1. Field of the Invention
The present invention relates to a cooled turbine blade and a combined cycle power plant having a gas turbine with this cooled turbine blade.
More specifically, the present invention relates to a cooled turbine blade in which a cooling medium flow is improved. This blade is suitably cooled with steam. The gas turbine having this blade is used in a combined cycle power plant, and the blade is cooled by the steam generated in this plant, and the steam which cooled the blade is recovered and supplied to a steam turbine.
2. Description of the Related Art
In a gas turbine, when a turbine gas temperature of a main gas is increased, thermal efficiency can be improved. For this reason, the gas turbine is designed to maximize the turbine gas temperature. For this purpose, heat resistance of the turbine blade must be improved. A flow path for a cooling medium is formed inside the blade, and the cooling medium such as a cooling gas flows through the flow path. The blade is cooled by the cooling gas which flows through the flow path, thereby improving the heat resistance of the blade.
FIG. 1 is a cross-sectional view of a conventional cooled turbine blade of this type. The blade shown in FIG. 1 is a stator blade in a turbine. This blade is made of a super alloy, and cavities are formed inside the blade. A partition wall 203 is formed at the central portion inside the blade in a direction of chord, and the partition wall 203 continues in a direction of span. A plurality of openings 205 are formed in the partition wall 203. The internal space of the blade is partitioned into fore and aft cavities by the partition wall 203. These cavities extend in the direction of span and are open to at least one tip. A fore insert 201 and an aft insert 204 are inserted into the cavities from the tip. These inserts 201 and 204 are hollow cylindrical members. Many impingement holes 202 and 206 are formed in the wall surfaces of the inserts 201 and 204, respectively. A plurality of small-diameter paths 207 are formed at a trailing edge of the blade in the direction of chord. The leading ends of these paths 207 communicate with the aft cavity, and their trailing ends are open to the trailing edge of the blade.
A cooling ga such as air is supplied to the inserts 201 and 204, and this air is sprayed from the impingement holes 202 and 206 and collides against the inner wall surface of the blade to cool the wall of the blade. The air sprayed from the insert 201 flows in a space between the outer circumferential surface of the fore insert 201 and the inner wall surface of the blade in the direction of chord. The air then flows in the aft cavity through the openings 205 formed in the partition wall 203 and is the mixed with air sprayed from the aft insert 206. The mixed air flows between the outer circumferential surface of the aft insert 206 and the inner wall surface of the blade in the direction of chord and is discharged into the main gas from the trailing edge of the blade through the paths 207.
This conventional turbine blade has the following drawback. The gas flowing between the outer circumferential surfaces of the inserts and the inner wall surface of the blade crosses the gas spraying from the impingement holes of these inserts to set a crossed flow state, thereby degrading the cooling effect of the blade wall.
The cooling gas sprayed from these inserts is discharged into the main gas and is mixed therewith. The temperature of the main gas is decreased to degrade turbine efficiency. For this reason, the cooling gas cannot have an excessively high flow rate, and sufficient cooling cannot be performed.
Use of steam as the cooling gas has been taken into consideration. The steam has a larger specific heat than air, and a cooling effect can be improved. When this steam is discharged in the main gas, however, a decrease in temperature of the main gas is increased due to a large specific heat of steam, resulting in inconvenience.